The present invention relates to a push button switch covering member made from a silicone rubber and having a protective coating layer on the surface thereof and a method for the preparation thereof. More particularly, the invention relates to a push button switch covering member of silicone rubber provided at least on the outwardly facing surface thereof with a coating layer to serve for protection against stain and abrasive wearing as well as a reliable method for the preparation thereof.
A push button switch covering member, referred to simply as covering member hereinafter, made from a rubber or, in particular, silicone rubber has a structure consisting of a base plate and a dome-like raised portion of a relatively small thickness having a riser part rising on the base plate and a flat key top. Such covering members are widely used in various kinds of electric and electronic instruments as an essential constituent of a push button switch consisting of a substrate plate on which an electrode as the fixed contact point is provided and a covering member mounted on the substrate plate which is provided with a movable contact point on the lower surface of the key top at a position to face the fixed contact point on the substrate plate. When the key top of the dome-like portion is pushed down with a finger tip and the like, buckling of the riser part is caused so that the movable contact point on the lower surface of the key top is brought into contact with the fixed contact point on the substrate plate so as to close the electric circuit.
As a modern design of such a covering member, the key top of the covering member generally made from a light-shielding rubber is provided with a light-transmitting area to serve as indicia to exhibit a letter, numerical figure, sign and the like indicating the function of the particular push button switch with a lighting means inside of the dome-like portion of the covering member to illuminate the light-transmitting indicia from inside so that the push button switch can be readily recognized even in a very dark environment. For example, the covering member as a whole is made from a transparent rubbery material and the outer surface thereof is coated with a light-shielding ink or paint compounded with carbon black or titanium dioxide filler leaving certain areas of the key top in the form of the desired indicia. Several modifications have been proposed and are under practical use for such a covering member having indicia in the key top having visibility even in a dark place.
When such a covering member having a light-shielding coating layer on the outer surface thereof is used for long-term service by pushing many times with a finger tip, a trouble is unavoidably caused that the light-shielding coating layer is abrasively worn out by rubbing with finger tips, clothes of the switch operator and dust particles having a high hardness such as sand particles deposited thereon so as to greatly decrease the recognizability of the indicia provided on the key top if not to mention the degradation in the aesthetic value of the device or instrument as a whole.
Proposals and attempts of course have been made to solve this problem. For example, Japanese Patent Kokai No. 4-96942 proposes to coat the surface of the covering member having a light-shielding coating layer with a non-silicone organic resinous material to form a protective layer. This method, however, has a disadvantage that, especially when the underlying light-shielding coating layer is made from a silicone-based coating composition compounded with a large amount of carbon black, the adhesive bonding between the underlying light-shielding coating layer and the protective overcoating layer cannot be high enough as a consequence of the unique surface properties of silicone materials in general usually exhibiting surface releasability. Although the adhesive bonding strength between layers in general can be improved by the use of a so-called silane coupling agent as a primer, no silane coupling agent is known which is fully effective for the combination of the resinous materials in this case if not to mention the problem inherent in the use of an amino group-containing organosilane compound as the primer due to eventual yellowing in the long-run use of the covering member.